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Population growth & regulation.
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One foundational idea in ecology is that, when given plentiful resources and an absence of controls, populations grow rapidly. They go through what is called exponential population growth. This is true of basically all populations- humans, and bees, and skunk cabbage, etc…
Population growth is mathematically modeled- check out the book if you want the calculus. The main idea is that r is the rate of population increase. When there are more births than deaths in a population, r is positive. When births and deaths are balance- it is 0, when more deaths than births, it goes negative. R is sometimes called the intrinsic rate of population increase, and is associated with exponentially increasing populations… thus “r-selected” is really referring to a species adapted to rapid population growth.
reindeer or caribou Latin name = (__________ ________)
reindeer or caribou Latin name = (Rangifer tarandus). This is an example of “exponential growth” the population has encountered a situation where resources are plentiful and there are no predators to regulate growth. Note that this is the situation with white tailed deer in Ohio! If you introduce a predator this curve does not do this. Take home point, births strongly outweigh deaths, population is skyrocketing.
whooping crane- was driven nearly to extinction. Down to 21 birds total! Their has been a massive conservation effort- and the population is responding like you would expect for a species being protected from population regulation. http://www.youtube.com/watch?v=XiikddBUOQs&NR=1 http://www.youtube.com/watch?v=fQ7-nSpZMDE&NR=1
Population Ecology is about to impact your life- whether you like it or not.
Exponential population growth is a concept that explains a condition that normally does not occur in nature. Resources are limited! Concept of “carrying capacity.”
As a population grows, competition for (limited) resources increases. • As the population demands on resources approach the ability of the habitat to provide resources (K- carrying capacity), the growth rate levels off • This is the logistic (rather than exponential) model of population growth. • Populations can exceed K at times, but usually crash afterward (reindeer example). • But even the logistic model, is based on single species interactions in a constant environment….
Exponential growth is useful in some instances, but does not apply in most natural systems In most populations in (relatively) undisturbed systems, the population growth rate is dynamic, through time and space. Stochastic processes intervene in population growth. (think- rockslide) Populations are influenced limited resources. (density dependent regulation- logistic growth) Populations are influenced other species (interspecific competition) Populations are influenced by predators (& prey), mutualists, parasites, pathogens, etc.
A life table is an accounting method used to track population numbers though assessment of survivorship year to year. You start out with a particular number of individuals (eg, 530, in the table). Then you track how many die each year to calculate a mortality rate. The mortality rate (qx from life table) is simply the number of individuals who have died divided by the total number of individuals at the start of the time period. changes through time. (e.g., in year 1, 79/159 = 0.5. In other words, half of the individuals died during that time period. If you plot qx through time you get a good idea of the mortality pressures over the life of the study organism.
The mortality rate (qx from life table) is very often life-stage dependant. Individuals are more vulnerable at some stages than others. This is useful, especially for managers. For instance, if your goal as a natural areas manager is to restore a particular plant that has been lost. You have to decide whether to plants seeds, or seedlings, and if seedlings- how old should they be? In this example, you might choose to plant rosettes.
Mortality Survivorship The “transpose” of the mortality curve, of course, is a survivorship curve. This gives an idea of how many individuals make it each year, and ergo, the mortality pressures on that organism. The shape of the survivorship curve matters. Different shapes are associated with different life history patterns. Organisms that have relatively uniform survivorship across there life would have a curve like this one for squirrel. There is year to year variation, but overall relatively smooth,
Mortality Survivorship In contrast, an organism like this sedum does terribly through the germination and establishment phase! From July to December almost everyone dies. From then on, though, the curve is pretty flat. The organism has a maximum life span, so there is mortality, but once you get past the rosette phase, survivorship is strong
Red deer survivorship curves. This organism has very strong survivorship the first few years of live- then- especially for males, the survivorship falls off rapidly. This is evidence of maternal investment in offspring. Once the deer heads out on its own, mortality rates go up.
Survivorship curves have been categorized into three broad shapes. • Type I curve, early survivorship, mortality increases when the organisms reaches old-age. (humans, large mammals) • Type II curve, constant mortality/survivorship. (rodents, birds, some plants) • Type III curve, early mortality, if you make it to maturity then survivorship is high. (trees, fish, invertebrates)
Although we generally think population growth is related to resources, competition, predation, etc… there are some factors- unrelated directly to resource issues that have a significant influence on long-term population growth. We have assumed thus far a deterministic process for population growth. The numbers react to the resources and to life history limitations. But, there are other forces in play… Demographic stochasticity- is caused by random variation in birth and death rates. In reality, mating, reproduction, and death, is not completely orderly and predictable. Random factors can intervene, driving the population numbers in ways that cannot easily be predicted. Environmental stochasticity- is caused by random variation in birth and death rates due to changes in environmental conditions. Drought, etc.
Population unpredictability is especially important in small populations. Small populations are vulnerable to extinction and loss, partly because there are just smaller numbers- but also because of some specific ways these populations behave. You cannot assume that you can go from a small population to a big on, just based on resource availability. Allee effects- are population limiting factors related to birth and death rates specifically associated with small populations. For Panex as the population numbers go down, so do the number of fruits produced per plant. WHY? They should have MORE resources if there are fewer plants. Death rates can also be influenced- if organisms use gregarious behavior to avoid predators, for instance. Numbers could be below a particular threshold. Also- smaller numbers = less genetic diversity and the possibility of imbreeding depression.